Messages in this thread | | | Date | Sun, 18 Apr 2010 14:54:52 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 02/16] rcu: make dead code really dead |
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oN Sun, Apr 18, 2010 at 02:12:16PM -0700, Josh Triplett wrote: > On Sun, Apr 18, 2010 at 06:42:17AM -0700, Paul E. McKenney wrote: > > On Sat, Apr 17, 2010 at 08:53:09PM -0700, Josh Triplett wrote: > > > On Sat, Apr 17, 2010 at 06:12:13PM -0700, Paul E. McKenney wrote: > > > > On Fri, Apr 16, 2010 at 09:53:27PM -0700, Josh Triplett wrote: > > > > > On Fri, Apr 16, 2010 at 03:29:27PM -0700, Paul E. McKenney wrote: > > > > > > On Fri, Apr 16, 2010 at 02:16:10PM -0700, Josh Triplett wrote: > > > > > > > On Fri, Apr 16, 2010 at 07:23:48AM -0700, Paul E. McKenney wrote: > > > > > > > > On Thu, Apr 15, 2010 at 04:52:52PM -0700, Josh Triplett wrote: > > > > > > > > > On Thu, Apr 15, 2010 at 11:13:25AM -0700, Paul E. McKenney wrote: > > > > > > > > > > From: Lai Jiangshan <laijs@cn.fujitsu.com> > > > > > > > > > > > > > > > > > > > > cleanup: make dead code really dead > > > > > > > > > > > > > > > > > > > > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> > > > > > > > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > > > > > > > > --- > > > > > > > > > > kernel/rcutree.c | 4 ++-- > > > > > > > > > > 1 files changed, 2 insertions(+), 2 deletions(-) > > > > > > > > > > > > > > > > > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > > > > > > > > > index e54c123..6042fb8 100644 > > > > > > > > > > --- a/kernel/rcutree.c > > > > > > > > > > +++ b/kernel/rcutree.c > > > > > > > > > > @@ -1236,11 +1236,11 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed) > > > > > > > > > > break; /* grace period idle or initializing, ignore. */ > > > > > > > > > > > > > > > > > > > > case RCU_SAVE_DYNTICK: > > > > > > > > > > - > > > > > > > > > > - raw_spin_unlock(&rnp->lock); /* irqs remain disabled */ > > > > > > > > > > if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK) > > > > > > > > > > break; /* So gcc recognizes the dead code. */ > > > > > > > > > > > > > > > > > > GCC's new __builtin_unreachable would help here, though obviously we > > > > > > > > > can't count on 4.5 or newer quite yet. A wrapper in compiler.h would > > > > > > > > > let us use it when available though. > > > > > > > > > > > > > > > > So at some time when we can count on gcc 4.5 or newer, the code > > > > > > > > would look something like the following? > > > > > > > > > > > > > > > > if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK) > > > > > > > > this_is_unreachable(); > > > > > > > > > > > > > > Yes, exactly. > > > > > > > > > > > > > > > I suppose that in the meantime one could supply the code to use > > > > > > > > in the unreachable case: > > > > > > > > > > > > > > > > if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK) > > > > > > > > this_is_unreachable(break); > > > > > > > > > > > > > > > > But this is beginning to seem a bit strained to me. ;-) > > > > > > > > > > > > > > I'd suggest spelling that this way: > > > > > > > > > > > > > > if (RCU_SIGNAL_INIT == RCU_SAVE_DYNTICK) { > > > > > > > unreachable(); > > > > > > > break; > > > > > > > } > > > > > > > > > > > > > > But in any case, all of these do seem excessive just to avoid the need > > > > > > > for an ifdef. :) > > > > > > > > > > > > Actually, the "if" condition is a comparison of numerical constants, > > > > > > so no #ifdef is required. > > > > > > > > > > I just meant that those constants get #defined based on CONFIG_NO_HZ, so > > > > > an #ifdef on that would remove the need for the special handling of dead > > > > > code. > > > > > > > > True, I could put a #ifdef CONFIG_NO_HZ around that leg of the switch > > > > statement. Or am I still missing your point? > > > > > > No, you got it exactly. Hence my suggesting that all the other > > > alternatives (the if with a break, or with __builtin_unreachable) seemed > > > excessive just to try to convince the compiler to infer what an ifdef > > > would tell it explicitly. :) > > > > Which is exactly the purpose of the "if" statement comparing the two > > constants, right? ;-) > > Right, which also seems excessive compared to an ifdef, since it serves > the same purpose but more confusingly. ;)
Well, I must confess that it confused me when I added the spin_unlock()...
Thanx, Paul
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